Hover Picture Codes?

I want to be able to do what this forum does:
http://rpg-haven.forumotion.com/

When you put your mouse over a category, a picture pops up. How do you do that?!

I had added this on my test forum a few days back for a user.
http://troubleshoot.forumotion.net/

You can add such a code in the Forum description.

Code:

<div class="fmthumbnail">The forum to request for anything and everything of the RPGH specific. Custom avatars or siggies, group/guild membership, or request for missions here as well.<span><img src="http://i47.servimg.com/u/f47/14/07/41/67/94162911.jpg"><br></span></div>

Also, add this to your css.

Code:

.fmthumbnail span {
    position: absolute;
    background-color: gray;
    padding: 5px;
    left: -1000px;
    border: 1px black;
    visibility: hidden;
    color: black;
    text-decoration: none;
    }

    .fmthumbnail:hover span {
    visibility: visible;
    top: 100;
    left: 250px;
    }

it works! thanks so much!!!